Leaching is practiced with a variety of relatively low grade ores for extracting metal values where other techniques may not be feasible or economical. Broadly, leaching involves contact of crushed ore with a leaching solution that selectively attacks the metal bearing constituents for dissolving them and has limited attack on the other constituents of the ore. Leaching is commonly used for copper, uranium, and other ores having relatively low amounts of high value metals.
Typically, the metal values are recovered from the leach solutions by solvent extraction. Solvent extraction involves contact between the metal bearing aqueous leach liquor and an organic solvent that is not miscible with the aqueous phase. The organic phase contains a chelating agent that selectively combines with the desired metal so that its concentration in the organic phase increases and its concentration in the aqueous phase decreases. High rates of metal removal from the aqueous phase can be obtained in two or three stages of solvent extraction. The metals are then removed from the organic phase, typically by acid contact, and recovered as salts or metals. The aqueous raffinate from the solvent extraction may be recycled in the leaching process or discarded to avoid build-up of undesirable contaminants.
The principal methods of leaching metal ores at present include heap, in situ, vat and agitation leaching. Each of these techniques has inherent limitations in terms of efficiency, yield, or capital cost.
Heap and in situ leaching have some similarities. In each case the ore is relatively uncrushed and is in layers from about three to a few hundred meters thick. In in situ leaching the permeable ore remains in a relatively impervious cavity in the ground and leach liquid is percolated through it. In heap leaching the ore is piled in heaps as much as 100 meters deep and the top surface is sprinkled or flooded with leaching solution. These solutions percolate slowly downwardly through the bed and out of the bottom.
In both in situ and heap leaching efficiency is relatively low and the percentage yield of metals from the ore can be low. Metal recoveries are seldom more than about 65%. Factors involved in the low yield include the coarse size of uncrushed ore particles which limits penetration of the leaching solution and leaves the centers of particles essentially unleached; non-uniformity of ore contact by the solution due to channelling within the heap or in situ ore body; uneven distribution of leaching solutions on the surfaces of the heap or in situ ore body; non-uniformity of leaching reagent contact with the ore due to high consumption of the leaching reagent at the top of the heap, resulting in lean leaching solutions further down; loss of metal values or solutions through metal salt recrystallization in dead zones in the ore heap or ore body; losses of solution into ground water and other aquifers; and the like. In many heap leaching processes, the top portions are overleached and the lower portions are underleached.
Vat leaching is the most prevalent technique currently in vogue for treating crushed ores. According to this technique the crushed ore is placed in a large concrete vat and subjected to slow upward percolation of leach solution. Appreciable difficulty can be encountered in such leaching due to poor contact between the ore and leaching solution when excessive fine particles are present. The capital and operating costs for vat leaching are also higher than involved in heap or in situ leaching. Problems of non-uniform contact due to channelling and the like are less pronounced than in heap leaching but may still be present.
When high value ores are being treated, agitation leaching can be used. High operating costs and capital investment are limiting factors. In such a technique crushed and ground ore is agitated in an aqueous leach solution. Dissolved values are rinsed from the leached ore by countercurrent decantation. Such a technique is seldom feasible for low value ores because of the large volumes of material that must be handled. Agitation leaching is also sometimes known as slurry leaching.
It is therefore highly desirable to provide a technique for leaching that can be used on relatively coarsely crushed ores economically and with a high yield. In practice of this invention this can be accomplished by, for example, initially curing the crushed ore, at least 25% of which has a particle size greater than about 4 mesh, for two days or so in contact with a strong acid solution. This curing indurates the ore and solubilizes the metal. A thin layer of this ore in the range of about one-half to one meter thick then has a leach liquid percolated therethrough into a permeable substrate to extract metal values. Leaching of the thin layer employs progressively leaner leach liquids to effectively be countercurrent.
It is believed that the most pertinent prior technique is referred to in a paper entitled "Some Recent Developments in the Extraction of Uranium From Its Ores" by S. E. Smith and K. H. Garrett, in The Chemical Engineer, December 1972, pages 440 to 444, a copy of which accompanies this application. According to this paper, the strong acid process involves reduction of the ore particle size to minus one to two millimeters. The ore is contacted with around 10% strong acid solution and cured at elevated temperature (95.degree. C.) for about 18 hours. The solublized uranium is washed from the ore by alternative techniques including vat percolation, reslurrying, or "in a shallow bed by methods similar to those used for washing filter cake". The nature of this process is not further described and if the same as filter cake it would appear to involve only about five to fifteen centimeters thick through which liquid is percolated with or without vacuum assistance.
In practice of this invention it is found important that at least about 25% of the ore has a particle size greater than about 4 mesh (4.7 millimeters) and that the thin layer is in the range of about one-half to one meter thick. Other aspects also differ appreciably from anything suggested by the Smith paper.
Laboratory tests have been conducted to simulate heap leaching. Some of these tests have used narrow columns of ore in the order of 60 centimeters tall through which a leach liquid was percolated. Such tests are used to explore interaction of leaching solutions with various ore samples. Attempts are made to extrapolate such laboratory tests to full scale heap leaching with tall leach heaps, and, in general, the results are appreciably below expectations. It has not previously been recognized that it is important to have a thin layer for leaching rather than a thick heap piled as high as feasible.